High temperature catalysts, e.g. three-way catalysts (TWC), have utility in a number of fields including the abatement of nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbon (HC), such as non-methane hydrocarbon (NMHC), pollutants from gasoline-fueled internal combustion engines, such as automobile and other gasoline-fueled engines. Emission standards for unburned hydrocarbons, carbon monoxide, and nitrogen oxide contaminants have been set by various governments and must be met, for example, by new automobiles. To meet such standards, catalytic converters containing a TWC are located in the exhaust gas line of gasoline-fueled internal combustion engines. Three-way conversion catalysts are polyfunctional because they have the ability to substantially and simultaneously catalyze the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.
A TWC exhibiting good activity and long life comprise one or more platinum group metals (PGM), e.g., platinum, palladium, rhodium, ruthenium, osmium, and iridium. These catalysts are combined with a high surface area refractory oxide carrier. The refractory metal oxide can be derived from aluminum, titanium, silicon, zirconium and cerium compounds, resulting in the oxides with the exemplary refractory oxides including at least one of alumina, titania, silica, zirconia and ceria. Generally, the TWC are carried by gamma-alumina.
The TWC is deposited on a suitable substrate such as a monolithic material comprising a refractory ceramic or metal honeycomb structure, or refractory pellets such as spheres, beads or short, extruded segments of a suitable refractory material.
High surface area refractory metal oxides are often employed as a carrier for many of the catalytic components. For example, high surface area alumina materials, also referred to as “gamma alumina” or “activated alumina,” used with TWC catalysts typically exhibit a BET surface area in excess of 60 m2/g, and often up to about 200 m2/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa, and theta alumina phases. Refractory metal oxides other than activated alumina may be utilized as a carrier for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha-alumina and other materials are known for such use. Although many of these materials have a lower BET surface area than activated alumina, that disadvantage tends to be offset by the greater durability of the resulting catalyst.
Exhaust gas temperatures can reach 1000° C. or higher in a moving vehicle, and such elevated temperatures can cause activated alumina, or other carrier materials, to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage especially in the presence of steam. During this degradation, the catalytic metal becomes sintered in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity.
To prevent sintering of catalytic metals, alumina carriers are often doped with a stabilizing material. The stabilization of TWC catalyst carriers are known in the art. For example, U.S. Pat. No. 4,171,288 discloses a method to stabilize alumina carriers against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia, or strontia, or rare earth metal oxides such as ceria, lanthana, and mixtures of two or more rare earth metal oxides.
In a three-way catalytic converter, the air/fuel ratio (A/F) needs to be maintained within a narrow range to achieve high conversion efficiency for CO, HC, and NOx simultaneously. However, since a typical gasoline engine runs with the A/F oscillating in a certain range, an oxygen storage component is needed to counteract the effect of oscillating gas compositions in the exhaust by taking up oxygen during “lean burn” (high A/F) and releasing oxygen during “rich burn” (low A/F). Oxygen storage materials can store or release oxygen depending on the conditions that the materials are in. With current TWC, the addition of auxiliary catalyst material having oxygen storage capacity (OSC) mitigates A/F variation and adjusts the atmosphere at the surface of the catalyst, thereby controlling NOx discharge. Rare earth oxides, ceria more particularly, are commonly used as the primary component of oxygen storage materials.
It would be desirable to provide improved catalyst materials including carriers for PGM catalysts that exhibit good stability at high temperatures.